This invention relates to a manufacturing method for solar cell arrangements and more particularly to solar cells having protection against reverse current through them.
In one type of solar cell (or photovoltaic cell), a voltage is developed across a p-n junction in a semiconductor when it is irradiated by photons. A plurality of solar cells may be connected together in an array to provide a power supply, such arrangements commonly being used in spacecraft spaced and satellites for example. During operation, there may be circumstances where a single solar cell is shadowed or is faulty, and therefore not producing power, whilst the remainder of a sting of cells with which is connected in series is illuminated and producing power. This may lead to current being driven in reverse through the shadowed solar cell. Power is dissipated within the cell which is proportional to the reverse voltage across it. If the power density is sufficiently high, the cell may irreversibly break down.
One way in which this problem may be alleviated is to use a protection diode wired across one or several solar cells in reverse parallel with them. If one of the cells across which it is connected is driven into reverse bias, the protection diode diverts current from the cell. This limits the power dissipation in the shadowed cell by both limiting the voltage across the shadowed cell and by passing the reverse current.
The present invention seeks to provide an improved method of manufacturing solar cell arrangements which is particularly advantageous when applied to GaAs solar cells and cascade solar cells, although other types of solar cell may also benefit from it.
According to the invention, a method of manufacturing a solar cell arrangement includes the steps of:
forming a p-n junction in semiconductor material; separating the semiconductor material along a plane across the p-n junction to give an off-cut and a main body;
electrically connecting the off-cut in reverse parallel across a main body such that the off-cut forms a protection diode for a solar cell comprising that main body; and
providing first and second electrically conductive contacts on the front and rear respectively of the semiconductor material.
A method in accordance with the invention enables a protection diode to be readily manufactured at the same time that the solar cell itself is fabricated without requiring a large number of additional steps. In addition, because the diode is an off-cut of the main body which forms the solar cell, if the diode is placed adjacent the main body, being either spaced therefrom or mounted on the side of the main body, it closely conforms to the shape of the main body at that location, giving a particularly compact arrangement. This is particularly advantageous where the arrangement is to be incorporated with others in an array.
In another embodiment, the diode is mounted on the front or rear of the solar cell. This may require that a coverglass subsequently bonded to the solar cell include a recess for accommodation of the diode. However, a diode produced by a method in accordance with the invention may have only a relatively small thickness and it may be possible to use a standard coverglass and conventional automatic glassing techniques.
The off-cut may be electrically connected to the main body from which it was separated or alternatively to another main body formed when a different off-cut was separated from it. For example, a plurality of solar cells and protection diodes may be produced by separating semiconductor material to provide a plurality of off-cuts and a plurality of main bodies. It is not necessary that each off-cut be specifically matched to the main body from which it was separated.
Advantageously, at least one of the first and second electrically conductive contacts is provided before the semiconductor material is separated. The contact or contacts may be formed so that in addition to being extensive over the main body, they also cover the region of the semiconductor material surfaces which will be included in the off-cut following the separation step. The contact or contacts may then be utilized in connecting the off-cut to the main body. Alternatively, the metallization step may be carried following separation but this may be less convenient.
Preferably, the semiconductor material is separated by cutting it. For example, this may be done mechanically using a cutting tool or by a laser. Alternatively, the semiconductor material might be separated by fracturing it after pre-scoring it along the desired line and then applying pressure, perhaps on a ridged support to complete the separation step.
The separation may carried out to give any convenient shape of noff-cut and remaining main body. For example, the off-cut may be triangular in plan view, circular, square, rectangular or an irregular shape.
The method may be used in the manufacture of GaAs solar cells, cascade solar cells or may be applied to other types of solar cell.